tty: Add support for auto-detecting serial console from SysFS

This patch adds support for a new TTY device keyword `@console` which
auto-detects the system console by reading the SysFS file:

    /sys/class/tty/console/active

Embedded systems with one of /dev/ttyAMA0, /tty/ttyMXC0 or the original
/dev/ttyS0, as their active serial console can now instead use a single
line to /etc/finit.d/getty.conf to start the built-in getty on the same
TTY as the kernel already runs on:

    tty [12345789] @console 115200 noclear

With no TTY defined Finit will either do nothing (default), or start a
fallback shell if enabled in the configure script.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/tty.c

@@ -45,12 +45,28 @@ static LIST_HEAD(, tty_node) tty_list = LIST_HEAD_INITIALIZER();
 
 static char *canonicalize(char *tty)
 {
+	char buf[42];
 	struct stat st;
 	static char path[80];
 
 	if (!tty)
 		return NULL;
 
+	/* Auto-detect serial console, for embedded devices mostly */
+	if (!strcmp(tty, "@console")) {
+		FILE *fp;
+
+		fp = fopen("/sys/class/tty/console/active", "r");
+		if (!fp) {
+			_e("Cannot find system console, is sysfs not mounted?");
+			return NULL;
+		}
+
+		if (fgets(buf, sizeof(buf), fp))
+			tty = chomp(buf);
+		fclose(fp);
+	}
+
 	strlcpy(path, tty, sizeof(path));
 	if (stat(path, &st)) {
 		if (!strncmp(path, _PATH_DEV, strlen(_PATH_DEV))) {
@@ -168,6 +184,8 @@ int tty_register(char *line, struct timeval *mtime)
 		if (!dev && !cmd) {
 			if (args[i][0] == '[')
 				runlevels = line;
+			if (!strcmp(args[i], "@console"))
+				dev = args[i];
 			if (!strncmp(args[i], "/dev", 4))
 				dev = args[i];
 			if (!strncmp(args[i], "tty", 3))